Guido of Arezzo (aka Guido Aretinus aka Guido da Arezzo aka Guido Monaco aka Guido d’Arezzo) was a Benedictine monk in Italy. He was also a highly regarded music theorist of the Medieval era and the inventor of modern musical notation that replaced neumatic notation.
The “do-re-mi” scale are actually syllables taken from the initial syllables of each of the first six musical phrases of the first stanza of the hymn “Ut queant laxis (Hymn to St. John the Baptist)”.
Ut queant laxis
Originally, the first note of the scale was “Ut” which was later replaced by “Do” inspired by the word Dominus (Lord).
When the octave was created, a name for the seventh note of the scale had to be found. Guido of Arezzo decided that initial letters of the last line of this same hymn would be used – Sancte Iohannes – and “Si” was added to the scale.